dot: rename label to key in print_packed_decls()
dot: rename label to key in print_packed_decls()

This is the node parameter key, which is more explicit and will prevent
confusion in the following commits.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl